1. Field of the Invention
The present invention relates to an apparatus for fabricating a hard carbon coating to be prepared as a surface protective film for a magnetic tape, an optical magnetic disc or the like, and a method of manufacturing such a hard carbon coating. Further, the present invention relates to an apparatus for manufacturing a magnetic disc medium having high durability and high recording density and excellent in productivity on a polymeric substrate material, and more particularly to an apparatus for manufacturing a protective film having functions of wear resistance and lubricating ability, which is industrially applied to an image equipment, an information equipment field and the like.
2. Description of the Related Art
Hitherto, there has been known a technique by which a diamond-shaped carbon coating is formed. The diamond-shaped carbon coating has a diamond structure and is also called a diamond-like carbon (DLC) film. Hereinafter, this carbon coating is referred to as "hard carbon coating".
The hard carbon coating is coated on a surface of resin or a polymeric film and can be used as a wear-resistant layer or a protective layer. When manufacturing the hard carbon coating, a film forming apparatus such as a plasma CVD apparatus shown in FIG. 2 is used. In the film forming apparatus shown in FIG. 2, a pair of electrodes 112 and 114 are disposed within a vacuum vessel 111. One electrode 112 is connected to a high-frequency power source 115 (generally, 13.56 MHz) whereas the other electrode 114 is grounded. An object or a substrate 113 on which a film is to be formed is disposed on the electrode 112 side to which a high-frequency power (voltage) is supplied. Also, although not shown, a supply system, an exhaust system for a reactive gas, and a matching device for supplying voltage are provided.
In the plasma CVD apparatus shown in FIG. 2, electrons are charged onto the substrate 113 and the electrode 112 (electrode opposed to the grounded electrode 114) connected to the high-frequency power source 115. Therefore, H.sup.+ -ions and H-radicals contributing to heightened quality of the film by action of self-bias collide with the object 113, thereby to prepare the carbon coating having the diamond structure.
The hard carbon coating thus prepared can be used as the protective film of the magnetic recording medium such as a magnetic tape or an optical magnetic disc, etc. Since these magnetic recording media are made of magnetic material, it is necessary to protect the media from being mixed with a foreign matter or being damaged. For example, a DC bias is applied in addition to high-frequency discharge, whereby a carbon coating in which pin-holes of 10.sup.2 to 10.sup.5 per mm.sup.2 are formed is prepared on a surface of the magnetic recording medium.
However, according to the experiments of the inventors, it has been ascertained that the hard carbon coating having the pin-holes therein lacks long-term reliability as a protective film because moisture is infiltrated into the pin-holes. Also, it has been ascertained that improvements in hardness and adhesion of the hard carbon coating and the prevention of generation of the pin-holes are not always performed together.
Recently, there is a tendency to heighten the density of the magnetic recording medium. As a conventional magnetic recording medium, there has been known a coating type in which .gamma.-Fe.sub.2 O.sub.3 powders, CrO powders, pure iron powders or the like which are used for an audio or video tape material are coated on a polymeric substrate material together with an abrasive material and a binder. Further, a high performance magnetic recording medium on which a metal magnetic material has been vapor-deposited is used.
Furthermore, there has been known a technique by which a coating mainly containing carbon (also called a carbon film, a DLC or a hard carbon film) is formed on a surface of these magnetic recording medium, thereby to obtain a surface protection, a wear resistance or a lubricating ability. In general, the coating mainly containing carbon is fabricated by the CVD method typical of the plasma CVD method.
In the typical plasma CVD method, a substrate is located on a high-frequency voltage supply electrode (cathode) side, and a self-bias formed in the vicinity of the cathode allows a high-hard film to be fabricated. In general, a carbon film with a high hardness cannot be fabricated on a grounded electrode (anode) side.
When a coating mainly containing carbon is formed using the plasma CVD method of the parallel plate type, an organic resin substrate constituting an object of the magnetic recording medium must be located on a cathode electrode side. The magnetic recording medium for high-density recording is generally obtained by vapor-depositing the metal magnetic material. Therefore, if such an object is in contact with the cathode electrode, the object constitutes a part of the electrode, and therefore a high-frequency electric field is leaked as a result of which discharge occurs in an undesired region. There is a high possibility that the organic resin film constituting the object is damaged by such discharge, causing a problem on the stability and reliability of production.
Furthermore, fabricating the hard carbon film which is a protective film at the same time as the roll to roll type magnetic layer fabricating process is impossible because the carbon film forming speed is low.